According to reports, Lily and Rowland will sing Tina Turner's 'River Deep, Mountain High', while Little Mix and Contostavlos will perform a medley of Alicia Keys's 'If I Ain't Got You' and 'Empire State Of Mind'.

Collins and Barlow are said to still be undecided on their duet track, but have been rehearsing a number of songs, including Take That's 'Never Forget' and 'Greatest Day'.

"It's going to be more pressured than ever," an insider told The Mirror. "The acts will be singing in front of 10,000 people - and with their mentors.

"The judges will be nervous as well because the song could be the difference between winning and losing.

"And on top of that, Simon Cowell will be watching from LA. So the judges are almost auditioning for their places on next year's show as well."